The origins of the Undead are so old no one knows whence they came or how they came to be.
For as long as there has been life, there has been death. But death is not certain. Not all who die stay dead. Being neither living nor dead, the Undead fear no one in combat. Only the holy blessings of the Sacred Order can return these restless ones to the dead permanently.
While maybe not magical in origin, they grow stronger from magic and prefer dwelling in fetid swamps where they build homes and can build grandiose Wizard Guilds.